Archive for August 2014
Edinburgh has lots of great pubs, and we went to one of my favourites, which is Teuchters in the west end. It always has Jarl which is (as we say in Scotland) a good ‘session ale’. I mention this purely in the shameless hope that the bar staff in Teuchters read this and provide me with a free ale or two.
After that sublime first sip of Jarl the Q&A session continued. I have decided to illustrate some of what we chatted about with a few relevant images. You had to be there to appreciate the artistry on the beer mats and scraps of paper….
Q. So why does this dyke intrusion you told me about not just force its way to the surface and erupt? Is something stopping it?
A. Well the magma in the dyke intrusion is quite ‘happy’ where it is. The magma doesn’t actually have a lot of energy to expend in breaking apart the crust. After all isn’t easier to break apart a rock with a sledgehammer than with a bag filled with gloopy concrete?
Q. I don’t know, I’ve never tried it.
A. Ah, sarcasm. Sup your pint and listen quietly.
The tip of a dyke at Askja volcano that was propagating laterally towards you (i.e. out of the page). Note the start of a ‘split’ in the tip of the dyke. As this widens the two cooler leaves of the dyke move apart and fresh magma squeezes out. It’s likely to be an episodic process (stop-start).
I like to think of magma as being a lazy beast as it will always move to where it’s easiest to do so. So all things being equal, if there’s no easy pathway to the surface the magma will just sit and stew and solidify within the crust. Nobody knows exactly what the crust above the dyke intrusion is like, but if for example it’s a stack of lava flows, then these horizontal slabs of solid rock form a formidable barrier (or lid) on top of the magma.
An example of rifted Iceland crust at Thingvellir. Notice that there’s no single fault but rather a set of faults linked to one or more major faults.
But in Iceland we have rifting and this means that the crust splits in a preferred direction. So there is a pervasive weakness in the Icelandic crust that is especially well developed in the active rift zones, and when a weakness develops and a crack happens to connect underlying magma to the surface, then you get an eruption. Driven by gas expanding and accelerating as the magma ascends to shallower crust and lower pressures. Remember bubbles?
Q. Sigh, you and the bubbles again. Why can’t the magma in your dyke intrusion just go where it wants?
A. If the crust near the dyke is weaker in one direction then this is where the magma will go. That’s why the dyke has been moving towards the NE so far. Now what actually happens around dykes in stress and strain terms is a tad complex, but let’s just say that the dyke wouldn’t have been able to form and propagate unless the crust was already weak in this area. And that the presence of the magma-filled dyke will influence the local stress field and favour some further weakening in the vicinity of the dyke. So although the regional stress field will largely dictate where the magma in the dyke intrusion can go, the dyke itself will have some influence in this.
Q. In simple terms now please? You know how garrulous and nerdy you get when you mix ale and enthusiasm.
A. Where the magma in the dyke intrusion goes is largely dependent on weaknesses in the local crust which are either there already and ready to part, or will appear as this event develops. The magma itself has a say in this, arguably a minor one.
Two dykes intruding fragmented basalt at Askja. The dykes have ‘wavy’ margins because the fragmented basalt was a bit ‘sloppy’ rather than being a brittle solid. Note the prominent chilled margins on the outside of the dykes – a sign that the fragmented basalt was also a tad wet.
Near where the pair of dykes in adjacent image are exposed. High up on Askja’s south caldera wall, looking westwards.
Q. So did you learn about Bárðarbunga from a hurried swotting-up as this event kicked off?
A. I already knew a lot about Bárðarbunga because I did my PhD on the rhyolite-dominated volcano c.100 km to the SW (Torfajökull) where there’s excellent evidence that basalt dyke intrusions in fissures from the NE (i.e. in the direction of Bárðarbunga) had forced their way into Torfajökull and triggered eruptions there of rhyolite (a more viscous and sticky magma type). The last one in c1477 was fairly benign, with minor explosions and two lovely rhyolite lava flows. One of which has a natural hot pool where once can sit and watch the Northern Lights. But I digress. The eruption prior to c.1477 took place in c.874 AD and this led to a powerful and explosive rhyolite eruption. The problem with explosive rhyolite eruptions is that it contains more gas than basalt (hence more bubbles) so it gets blasted apart more. And because rhyolite is less dense than basalt the rhyolite ash is less dense and can get transported further.
1477 AD rhyolite lava flow at Torfajökull (Laugahraun). Grey matter to left is older subglacial rhyolite eruption of Bláhnúkur.
Rhyolite lava flow erupted c.874 AD at Torfajökull (Hrafntinnuhraun). The author in 1983 inside a large bubble (vesicle for the pedants).
The area between Bárðarbunga and Torfajökull is one where massive fissure eruptions have occurred and from where some of the largest flood basalts in Iceland have poured forth. The two recent eruptions (c.1477 and c.874) weren’t as massive, but magma-water interactions with the big braided river to the SW area did produce strings of maars, tuff cones, explosion craters and so on, and consequently lots of fragmented basalt that dammed waterways and created temporary but large lakes.
Large braided rivers in the area between Bárðarbunga and Torfajökull. with subglacial basalt ‘Toblerone’ ridges
One of the flood basalts in the area between Bárðarbunga and Torfajökull. For scale is Professor John Smellie.
I’ve also worked at the Askja volcano to the NE of Bárðarbunga, and so have some idea of how a large basalt-dominated volcano with a large caldera like Bárðarbunga may have been constructed.
From Askja looking over to Kverkfjöll (left) and with the Dyngjujökull glacier to the right, currently considered the most likely place where meltwater from a subglacial eruption will pour forth from.
OK, time for a break while you buy me another ale.
Q. But I bought the first round!
A. Yes, but there’s no such thing as a free tutorial. And remember that my financial prudence has been enhanced considerably after some time living in Yorkshire….
The Science Media Centre (SMC) gathers information from scientists when relevant stories break. I provided them with their first update on Bárðarbunga and they asked me for an update this morning. Thought I’d share this with you.
Current situation at Bárðarbunga
Stable as at 10:30 GMT on 22 August 2014. No sign that an eruption is about to start.
Events on 21 and 22 August have raised anticipation (amongst some) that an eruption is imminent. Given the absence of escalation, these (i.e. summit earthquakes and slight subsidence) are best regarded as normal.
What’s happened so far?
“A magma filled fissure (dyke intrusion) some 25 km long has formed within the crust at 5-10 km depth. Sitting on top of this 25 km strip of crust is ice c.150-350 m thick. This dyke is on the NE flank of the main volcano, which is good news, as eruptions from beneath the main volcano itself have a higher probability of being powerful and explosive enough to generate sufficient fine ash to cause disruption to air traffic. There is no indication that the magma in the dyke is moving upwards, but if it did start moving upward this would heighten the possibility of an eruption.”
The NE flank zone – where magma is on the move
“Should an eruption occur from this flank dyke, the eruption style will be influenced by the presence or absence of ice above the eruption site, how much magma erupts, and the rate at which magma erupts. The likelihood of the magma currently in this dyke erupting to produce a substantial enough ash cloud to seriously affect international air travel is zero.
In summarising the flank dyke scenario, if this dyke grows at a similar (slow) rate to that of recent days, then it will either stall in the crust where it will cool and solidify, or it will gain access to the surface and erupt. A modest eruption is likely, with spectacular local explosions generated via interactions between magma and ice/water being observed unless the eruption is wholly covered by ice. Any subglacial eruption generates considerable amounts of meltwater as erupting magma can melt more than 10 times its own volume of ice (NB. variable – depends on conditions).”
“The authorities in Iceland have taken the precaution of evacuating everyone from an area where they would be cut off should a vital bridge be destroyed during a flood. The bridge crosses one of Iceland’s largest and most powerful rivers, and so authorities have alerted communities downstream of action they should take in the event of a flood. It should be noted that unlike the spectacular Amazon River sized flood following the subglacial Gjálp eruption of 1996, as there is no similar sub-ice topographic receptacle near the dyke intrusion in which to store meltwater till it escapes in one massive pulse, meltwater should escape rapidly and continuously from underneath the glacier which will help with managing and mitigating the effects of the flood.”
One worst-case scenario
“Although there are a number of ‘worst-case’ scenarios, one worth mentioning (because it is naturally on everyone’s radar because of the Eyjafjallajökull eruption) is a large and powerful explosive eruption from the main volcano itself that produces a sizeable ash cloud. I must stress that this is not even on the horizon at the moment – it’s somewhere well off the edge. Powerful and explosive eruptions from Iceland’s volcanoes are well documented, and there are many of them. Put simply, Icelandic magma contains enough gas to drive powerful explosive eruptions. The most recent unequivocal evidence of this was the 20 km high eruption plume produced during the 2011 eruption of Grímsvötn. Evidence from ash layers in Iceland indicates that powerful explosive eruptions have occurred in the past from Bárðarbunga.
“The good news is that if a powerful and explosive eruption does happen, then the experience gained during the 2011 Grímsvötn eruption (which involved a relaxation of the rules for flying with volcanic ash in the atmosphere), would result in a carefully managed strategy to minimise the number of flight cancellations and diversions. Despite erupting twice as much ash as Eyjafjallajökull 2010, flight cancellations during the Grímsvötn 2011 eruption were less than 1% of the number of flights cancelled during the Eyjafjallajökull 2010 eruption. An important factor in reducing the number of flight cancellations in 2011 was a wind direction that was favourable to UK and western Europe.
“In summarising the large and explosive eruption scenario, there are NO indications that this is about to happen. Even if it does happen we would not get a repeat of the disruption caused by the 2010 Eyjafjallajökull eruption, simply because if this same eruption happened tomorrow there would be far fewer flight cancellations (due to revised flight rules, better information on ash concentrations, and experience gained during 2010 and 2011).”
“Finally, volcanoes are complex natural systems, and when we know so little about a volcano such as Bárðarbunga because it hasn’t erupted in the modern era and thus we have no prior understanding of how it behaves when it stirs, it’s difficult to anticipate what might happen. If this current event does not last long then it will be a volcanic speed dating experience. If it lasts longer, then we may get to know Bárðarbunga’s volcanic personality a little better.”
Yesterday a colleague decided to hold a ‘mock’ interview with me during our lunch break, which she recorded and I’ve just written up and tidied up. You may find it informative.
Q. So Dave, stop the 50:50 stuff when asked ‘will it erupt’. What do you really think?
A. It’s still 50:50! Whether it will erupt or not depends on a number of factors, some of which cannot be monitored. So that people can better understand why predictions are so difficult let me list some:
The magma is sitting at depth in a vertical fissure and slowly moving NE. It’s a dyke intrusion.
A key question is whether new magma is joining the magma in the dyke. If not (or it’s just a small amount), then there is unlikely to be an eruption. It will stall and cool.
However should a fracture suddenly appear above the dyke, then the magma is going to move upwards, and then it’s more likely to erupt.
Because as it moves up, it will reach a level where any dissolved gases (mostly water) will stop being dissolved, expand dramatically and accelerate upwards, and ‘push’ the magma to the surface. This is actually how eruptions are powered – bubbles.
Another scenario is if magma keeps being pumped into the dyke. The dyke has a number of choices: use the extra energy to keep moving NE; expand by moving to the SW, or grow up and/or down.
Get the picture?
Q. Thanks Dave, and stop calling me Bubbles. Right, we all love an apocalyptic story, so what’s the worst case scenario?
A. Ah, well, there’s more than one with this particular volcano – sorry. But these are nowhere on the horizon at the moment. Here are three.
- This presently benign little dyke intrusion is the forerunner to the uprise of large packets of melt from below (from the mantle) and it suddenly turns into a Laki-type flood basalt eruption. There’s still controversy over how these massive eruptions are fed in Iceland, but they always occur in fissures, and they have to involve the mantle because we have no definitive evidence that 10s of cubic kilometres of melt are stored under each central volcano just waiting to erupt. A little puzzle to solve is why these flood basalts (if they are fed directly from the mantle) have ‘shallow’ pressure signatures, but this might just mean they spend enough time at shallow dept in transit to ‘equilibrate’ to lower pressures.
- This event triggers activity within the heart of Bárðarbunga, beneath the summit, where there’s almost certainly some melt and or mush (melt+crystals) stored. This could be all basalt, or there could be some more ‘sticky’ magma around, such as rhyolite. Evidence from ash layers in Iceland indicates that explosive basalt eruptions from Bárðarbunga do happen, and that they are powerful. The good news is – and myself and John Stevenson have said this many times – is that we have less to worry about if this happens because we’ve already had one – Grímsvötn 2011. So we know that fewer flights will be cancelled simply because the old “ash in the sky you don’t fly” rules no longer exist. Everyone is much better prepared for a big and powerful explosive eruption. I’ve seen a few geologists say things like “Icelandic magmas do not contain enough gas to drive powerful explosive eruptions”. This is utter rubbish, incorrect, and misleading. These are invariably geologists who lack a true understanding of Icelandic volcanism because they have done little or no research there.
- Probably the worst-case scenario for Iceland is that this leads to a massive volcano-tectonic event in the fissure system to the SW of Bárðarbunga, as this is where a number of large flood basalt eruptions have occurred. The hydroelectric power plants on the rivers near to this fissure system would be in trouble, and we know that in the past large ash piles have dammed the rivers. The abundant water in this area results in spectacular (but fairly local) explosions and a high production of fragments as the abundant river water cools the erupting magma.
Q.Final question. You mentioned over coffee that you’d been very active on Twitter trying to get what you called the ‘right information’ out there. But isn’t there a danger that others will pinch your work and re-cast it as their own?
A. That comes with Twitter territory. I’d much rather try and provide an informed and scientifically-based set of views and ideas that can be pillaged and re-used (usually without credit) than leave it to those who don’t understand Icelandic volcano-tectonics to mislead (not always deliberately I hasten to add). I appreciate it when folks give me credit, but I don’t expect it. If you are being paid from public money to do your science, then put your knowledge to good use for the benefit of the public. Getting credit for it is a bonus, not a right.
Q. OK – late for the next meeting Dave. Maybe continue with a pint or two later?
A. Only if it’s a real ale acceptable to my palate.
On the night of 23/24 July 2014 (around midnight) there was a large landslide in the SE corner of the steep inner wall of the 1875 AD caldera at the Askja volcano in central Iceland. This event is simply the latest (albeit large and spectacular) of many that have formed the current water-filled caldera of Öskjuvatn (Askja lake). It is part of the ongoing process of the formation of this youngest caldera at Askja, which is after all only 139 years old and which after its initiation in 1875 took several decades (until c.1932) to get close to the shape we see today.
This blog post contains images from before and after the landslide. I was fortunate to be doing fieldwork nearby (collecting samples from basalt subglacial mountains) when I heard that access to the ‘safe’ area above the lake had just been granted. So we went there on the first day that the area had been opened since the landslide. It was a bit special.
A. Taken in July 2011 – the site of the July 2014 landslide.
B. Taken in July 2011. Yellow line shows the major fracture system that was exploited during the July 2014 landslide.
C. Taken in July 2011. Purple shaded area shows roughly the part of the inner wall that collapsed during the July 2014 landslide.
D. Taken on 26 July 2014, 3 days after the landslide.
Estimates of the volume of the landslide range from 24-60 million cubic metres, and no doubt this will become refined as Icelandic scientists either gain access to the area or use digital elevation models to obtain more precise measurements.
The main hazard from the landslide was not the slide itself, as this occurred in a location well away from tourist trails. This location is visited only rarely by geologists utilising the superb exposures revealed by the caldera collapse to gain deeper insight into Askja’s past geological evolution. See Graettinger et al., 2013.
Nope, the main hazard from the landslide was the wave triggered by the sudden entry into the lake of a large mass of debris. Various people have called it a tsunami, a displacement wave, and a seiche. Tsunami will do, and estimates place it as 60-75 m high when it reached the opposite caldera wall there the vast majority of tourists gather to gaze over the lake and into the small crater of Víti (Hell) filled with turquoise coloured, warm, and sulphurous water. Fortunately nobody was in Víti at the time or they’d have had a shock (and an unwelcome cold shower) as the top of the tsunami wave spilled into Víti.
Figure E. The small water-filled crater of Viti, which lies just north of the rim of the 1875 AD (youngest) caldera at Askja. It was one of the vents of the 1875 eruption – the rest are buried beneath the lake water. ‘Spillover’ marks the low point where water from the tsunami wave poured into Viti.
The image below shows the raft of rhyolitic pumice and ice that remains after the landslide, with the source of the pumice being loose and unconsolidated deposits from the 1875 eruption. It will be interesting to see how long this raft persists, as the strong winds of Autumn and Winter will deposit much of the material on the eastern shore.
Figure F. Raft of rhyolitic pumice and ice occupying the northeast corner of Askja lake. Debris-covered areas clearly indicate inundation by the tsunami wave. Access to these areas to check extent of inundation was not possible as the area is closed.
Images from the landslide source – 2010 and 2011
Figure G. August 2010, at the eastern end of the headwall of the July 2014 landslide. looking to the west. Outcrops show downward movement relative to the ridge crest, and multiple parallel troughs indicating fault development. Some block rotation resulting in dip to the south (left) was apparent on closer inspection.
In 2010 and 2011 I was co-supervising a PhD student who was mapping the older rocks that lie on the east and south of the young 1875 AD caldera right down to the lower outer flanks of the volcano. I also visited the southeast corner with an Earthwatch group in 1985 and made the surprise discovery that there was an old rhyolite dome here, which I confirmed with a chemical analysis. It was apparent that the area around the top of the rhyolite dome and to the west was unstable and that a fault system had been active given that parts of the rhyolite dome had moved downslope and been rotated to dip 5-15 degrees to the south.
Figure H. From the eastern end of the 2014 landslide headwall, looking across the lake to the Viti crater.
To be honest, on the past occasions I was above the headwall of where the July 2014 landslide occurred (i.e. in 1985, 1987, 2010, and 2011) I was aware of the potential for a landslide in this area, but from the evidence I could see of other landslides (especially the older one immediately to the east) it looked like any future landslide may be a gentle slump rather than a headlong dash into Askja lake.
Figure I. Older landslide immediately to east of July 2014 landslide. This older one contains large intact blocks of rotated rhyolite lava from the dome above. Look carefully and you can see these be seen on images A-D above. In the foreground is one of the basalt vents from the 1920s, when a number of basalt (and mixed-magma) eruptions occurred around the 1875 caldera margins. This vent erupted a number of silicic lithics (non-juvenile clasts), some of which have chemical affinities to the old rhyolite dome nearby, whilst some suggest that other rhyolite sources lie buried.
Well the eastern fringes of where the July 2014 landslide occurred formed a convenient way up to the top in this area, but this has now gone. And the landslide has covered over more (if not all) of what was a poorly exposed 1920s basalt lava. The debris dumped onto the lovely little 1920s basalt lava of Bátshraun (0robably 1921) will have covered some of the exposures I was working on – which provide evidence of lava-ice/water interactions at the time of its eruption.
Figure J. Consequence. A straightforward route up to the rim at this point has now gone. It went up the eastern edge of what came down in July 2014.
Figure K. Consequence. Photo taken August 2010 shows a basalt lava flow from the 1920s which is now largely/wholly covered by debris from the July 2014 landslide.
Figure L. Consequence. Flow front of the 1920s Bátshraun basalt lava, showing typical a’a upper surface (to left) with glassy and block-jointed lava at lake level indicating more rapid cooling of the lower part of the lava flow. See Figure F for location (debris-covered lava).
Consequence. Figure M. Detail of blocky and glassy texture of Bátshraun basalt lava, showing pseudopillow fractures (long and curving with small joints perpendicular to main fracture). On right is actual pseudopillow fracture surface.
Working in this area one is aware of the regular small rockfalls from the steep north-facing wall of the 1875 AD caldera, and of the larger slumps that have taken place. As mentioned above I was surprised at the rapid displacement of lake water that let to such a dramatic tsunami wave being formed, but then I’m a volcanologist and not a landslide expert.
No doubt landslide experts will evaluate the potential for additional landslides from the zones adjacent to the July 2014 headwall, as these may have been weakened and potentially be ready to go. However these zones appear fairly small in comparison to the estimated c.800-900 m length of caldera wall that collapsed on 23/24 July.
There will be further landslides at Askja simply because the 1875 caldera is still ‘settling’ and will be for some time, with the southern and eastern caldera walls being likely sources because this is the area which underwent the largest amount of subsidence as a consequence of caldera formation (i.e. a sizeable chunk of pre-existing elevated terrain disappeared from the SE corner into the developing caldera). The southern walls of the caldera are particularly steep and consequently material shed from this area has a high probability of entering the lake and displacing water.
An interesting research project would be to look specifically for evidence of past tsunamis at Askja lake, to evaluate whether the July 2014 event was an extreme/low probability event, or just the latest in a number of larger events. My hunch (based purely on the pristine surface of the Bátshraun lava flow prior to this event that is now covered in debris – see Figure F) is that these larger events are infrequent.
The spectacular large landslide of 22/23 July won’t stop me working at interesting localities along the shoreline of Askja lake in future as the risk of a repeat seems very small (though this may change if the authorities carry out a more thorough examination of the source zone and say otherwise). At present the authorities are allowing access only to the relatively safe areas well above the lake level. It will be interesting to see whether this changes over the next few weeks.
In any case, Askja is a truly spectacular place to visit even if you don’t get to go down to the lake edge. And its dynamic nature has been superbly illustrated by this recent landslide, along with its effects and aftermath.